Low pressure discharge lamps, such as fluorescent lamps, include a tubular glass envelope which is usually linear or U-shaped. It has been found desirable to provide the glass tube with one or more grooves in order to increase the light output from the lamp. The use of such grooves is described, for example, in U.S. Pat. No. 4,582,523 issued Apr. 15, 1986 to Marcucci et al and U.S. Pat. No. 4,585,468 issued Apr. 29, 1986 to Marcucci et al. The grooves are formed such that both the inside diameter and the outside diameter of the glass tube are reduced in the groove area. Grooves may be also be formed in glass tubes for decorative purposes or for any other purpose.
The aforementioned Marcucci et al patents describe a groove-forming technique wherein a heated forming wheel is applied to the glass tube as it is rotated. The heated wheel can be used to form one groove at a time, or multiple, heated forming wheels can be utilized to provide multiple grooves simultaneously.
The technique which utilizes one or more heated forming wheels has various shortcomings. The forming wheel must be controlled at the proper temperature to avoid thermal cracks in the glass during grooving. Even if the wheel is maintained at a constant temperature, the glass must be slightly below its softening temperature at the edges of the wheel to insure proper groove shape. Because of the temperature gradient across the width of the groove, the glass is susceptible to thermal cracks. The temperature of the forming wheel must be a compromise between the optimum for the hottest area of the groove and for the relatively cool edges of the groove. If different shapes or groove dimensions are desired, new wheels must be machined, and the wheels on the groove-forming machine must be changed.
In addition, the forming wheel must rotate at the same speed as the glass tube when contact is made and throughout the groove forming process. It is difficult to control rotational speeds consistently in a production setting. The force and speed with which the forming wheel is pressed into the glass tube is critical because microcracks can be caused by excess pressure on the cooler groove edges. The pressure applied to the glass tube becomes more critical as the temperature gradient between the wheel and the glass increases. Thus, the glass is stressed both by pressure and thermal gradients. Finally, oxidation buildup from constant heating of the forming wheel requires special consideration with respect to wheel construction and maintenance.
It is a general object of the present invention to provide an improved method for forming a groove in a glass tube.
It is another object of the present invention to provide a noncontact method for forming a groove in a glass tube.
It is a further object of the present invention to provide a method for forming a groove in a glass tube wherein damage to the glass tube is minimized.
It is yet another object of the present invention to provide a method for forming a groove in a glass tube wherein the dimensions and shape of the groove can be changed without retooling.
It is still another object of the present invention to provide a method for forming a groove in a glass tube which is easily adaptable to high volume production.